This invention relates to an enhanced purification method by which a high-purity metallic indium feed with a purity of about 99.99% (4N) is further purified to give metallic indium with a purity of about 99.9999% (6N) or higher and which is also applicable for such enhanced purification of antimony, zinc, tellurium, magnesium, cadmium, bismuth and silver (which are hereunder referred to as similar metals). The invention also relates to an apparatus for purification that is used to implement the method.
Indium is generally produced as a minor amount component of zinc concentrates, so in zinc metallurgy, it is recovered either as flue cinder or as a concentrate obtained in an intermediate step such as eletrowinning of zinc. In recent years, indium is also recovered in pure form from waste compound semiconductors. To purify the indium feed, three methods are commonly used and they are electrolysis, vacuum distillation and zoning.
The metallic indium obtained by electrolysis or vacuum distillation is about 99.99% pure and contains at least 0.5 ppm each of impurities such as Si, Fe, Ni, Cu, Ga and Pb. The purification from waste compound semiconductors has the problem that large equipment and prolonged time are needed to separate and recover indium.
In the zone purification method, the purified indium mass has to be cut and there is a potential hazard of contamination; hence, the purification process inevitably suffers limited throughput and lowered yield. In addition, when the purified indium is cast into an ingot, impurities may enter during casting to cause contamination.
With a view to solving these problems, the present inventors previously developed an improved technology for purifying indium to a purity of at least 99.9999% by vacuum distillation and proposed it in Japanese Patent Application No. 8-294430. As it turned out, this technology had the problem that purification became more difficult as the difference between the vapor pressures of the metal of interest and impurity elements decreased. Hence, it was desired to develop a purification technology that was capable of producing indium of higher purity with higher efficiency and which was also applicable to the similar metals mentioned above.
An object, therefore, of the present invention is to provide an enhanced purification method by which even an indium feed containing many impurity elements can be purified consistently and at high speed to a purity of 99.9999% or higher and which is also applicable to the above-mentioned similar metals to yield equally purified products.
Another object of the invention is to provide an apparatus for purification that can be used to implement the method.
The present inventors conducted intensive studies in order to attain the stated objects by a two-step process in which the indium in an indium feed was evaporated and then condensed for recovery in the first thermal purification step to be separated from impurity elements of lower vapor pressure and in which the recovered indium was then heated in the second thermal purification step to evaporate away impurity elements of higher vapor pressure. As a result, they found that not only the impurity elements having lower vapor pressure than indium but also those having higher vapor, pressure could be separated in a consistent and efficient manner to yield indium with a purity of about 99.9999% or higher. They also found that by using graphite as the constituent material of areas which were to be contacted by indium during the purification process, in particular, the inner tube and by providing diffuser plates in the pathway of distillation in the second thermal purification step, recontamination could be prevented and the purification speed could be markedly improved. The inventors also found that this technology was applicable not only to indium but also to other metals that could be purified by the difference in vapor pressure, in particular, the similar metals mentioned above.